The weather is equally variable in Pennsylvania during the greatest part of the winter. The mercury fell from 37° to 4-1/2° below 0 in four and twenty hours, between the fourth and fifth of February, 1788. In this season nature seems to play at cross purposes. Heavy falls of snow are often succeeded in a few days by a general thaw, which frequently in a short time leaves no vestige of the snow. The rivers Delaware, Schuylkill, and Susquehannah have sometimes been frozen (so as to bear horses and carriages of all kinds) and thawed so as to be passable in boats, two or three times in the course of the same winter. The ice is formed for the most part in a gradual manner, and seldom till the water has been previously chilled by a fall of snow. Sometimes its production is more sudden. On the night of the 31st of December, 1764, the Delaware was completely frozen over between ten o'clock at night and eight the next morning, so as to bear the weight of a man. An unusual vapour like a fog was seen to rise from the water, in its passage from a fluid to a solid state.
This account of the variableness of the weather in winter, does not apply to every part of Pennsylvania. There is a line about the 41° of the state, beyond which the winters are steady and regular, insomuch that the earth there is seldom without a covering of snow during the three winter months. In this line the climate of Pennsylvania forms a union with the climate of the eastern and northern states.
The time in which frost and ice begin to show themselves in the neighbourhood of Philadelphia, is generally about the latter end of October or the beginning of November. But the intense cold seldom sets in till about the the 20th or 25th of December; hence the common saying, “as the day lengthens, the cold strengthens.” The coldest weather is commonly in January. The navigation of the river Delaware, after being frozen, is seldom practicable for large vessels, before the first week in March.
As in summer there are often days in which fires are agreeable, so there are sometimes days in winter in which they are disagreeable. Vegetation has been observed in all the winter months. Garlic was tasted in butter in January, 1781. The leaves of the willow, the blossoms of the peach tree, and the flowers of the dandelion and the crocus, were all seen in February, 1779; and I well recollect, when a school-boy, to have seen an apple orchard in full bloom, and small apples on many of the trees, in the month of December.
A cold day in winter is often succeeded by a moderate evening. The coldest part of the four and twenty hours, is generally at the break of day.
In the most intense cold which has been recorded in Philadelphia, within the last twenty years, the mercury stood at 5° below 0. But it appears from the accounts published by Messieurs Mason and Dixon, in the 58th volume of the Transactions of the Royal Society of London, that the mercury stood at 22° below 0, on the 2d of January, 1767, at Brandywine, about thirty miles to the westward of Philadelphia. They inform us, that on the 1st of the same month, the mercury stood at 20°, and on the day before at 7° below 0. I have to lament that I am not able to procure any record of the temperature of the air in the same year in Philadelphia. From the variety in the height and quality of the soil, and from the difference in the currents of winds and the quantity of rain and snow which fall in different parts of the state, it is very probable this excessive cold may not have extended thirty miles from the place where it was first perceived.
The greatest degree of heat upon record in Philadelphia, is 95°.
The standard temperature of the air in the city of Philadelphia is 52-1/2°, which is the temperature of our deepest wells, as also the mean heat of our common spring water.
The spring in Pennsylvania is generally less pleasant than in many other countries. In March the weather is stormy, variable, and cold. In April, and sometimes in the beginning of May, it is moist, and accompanied by a degree of cold which has been called rawness, and which, from its disagreeable effects upon the temper, has been called the sirocco of this country. From the variable nature of the weather in the spring, vegetation advances very differently in different years. The colder the spring, the more favourable it proves to the fruits of the earth. The hopes of the farmer from his fruit-trees in a warm spring are often blasted by a frost in April and May. A fall of snow is remembered with regret by many of them, on the night between the 3d and 4th of May, in the year 1774; also on the morning of the 8th of May, 1803. Such was its quantity on the latter day, that it broke down the limbs of many poplar trees. This effect was ascribed to its not being accompanied with any wind. The colder the winter, the greater delay we generally observe in the return of the ensuing spring.